Dispersion hardening of metal



United States Patent 3,399,086 DISPERSION HARDENIN G 0F METAL Dilip K. Das, Bedford, and George Freedman, Wayland,

Mass., assignors to Raytheon Company, Lexington, Mass., a corporation of Delaware No Drawing. Continuation of application Ser. No.

400,200, Sept. 29, 1964. This application Feb. 13,

1967, Ser. No. 615,846

1 Claim. (Cl. 148-32) ABSTRACT OF THE DISCLOSURE A copper alloy in the cold-worked annealed condition having a hardening oxide metal dispersion selected from the group consisting of aluminum oxide, chromium oxide, magnesium oxide and zirconium oxide, further characterized by having an elastic strength approximately twice that of the copper alloy in the oxidized dispersion-hardened condition.

This application is a continuation of application Ser. No. 400,200 filed Sept. 29, 1964, now abandoned.

This invention relates to the dispersion hardening of metal, and in particular to a novel procedure for the internal oxidation of dispersion-hardened metal alloys.

Many non-ferrous metals are relatively soft and lacking in mechanical strength unless alloyed with minor amounts of other metals or metallic compounds and/or are subjected to various hardening treatments. One such treatment which is referred to as precipitation hardening or age hardening involves the formation and precipitation of minute particles of intermetallic compounds throughout the matrix of the parent metal, the intermetallie compounds being formed by reaction between the parent metal and a very minor quantity of an alloying metal present therein. The tiny particles of the intermetallic compounds dispersed in the parent metal matrix contribute greatly to the strength and hardness of the parent metal. However, the intermetallic compounds tend to redissolve in the parent metal when the metal is heated to above the precipitation temperature of the intermetallic compounds involved. As the result, precipitation hardened metals tend to lose their strength and hardness when the metal is subjected to high temperatures such as that encountered in normal brazing or annealing treatment of the metal.

Another procedure for hardening non-ferrous metal is known as dispersion hardening, this procedure involving the uniform dispersal of minute particles of certain hardening metal oxides throughout the matrix of the parent metal. There are a variety of techniques for obtaining uniform dispersion of fine oxide particles in a metal matrix. The most common technique is that of powder metallurgy wherein fine particles of the metal oxide and parent metal constituents are uniformly mixed together in the desired proportions and then are pressed and sintered in the usual manner to obtain the desired dispersion hardened structure. However, this technique requires a large investment in pressing equiment and in sintering furnaces, and the possibility of non-uniform mixing of the powdered components and the porosity of the product obtained are important disadvantages. Another technique sometimes employed is to melt the parent metal and stir in the hardening metal oxide particles, the segregation effects of the metal oxide in the molten parent metal makes the process and the quality of the product difiicult to control. Yet another technique involves the simultaneous co-precipitation from a chemical solution of both the oxide of the parent metal and the oxide of the hardening metal. The resulting intimate mixture of co-precipitated oxides is then subjected to reducing conditions such that the oxide of the parent metal is reduced to that metal while the oxide of the hardening metal remains in oxide 3,399,086 Patented Aug. 27, 1968 form due to its greater thermodynamic stability. The resulting mixture of parent metal and hardening metal oxide is then pressed and sintered in accordance with conventional powder metallurgy techniques to form the desired dispersion hardened product. However, this technique is subject to the same limitations and disadvantages of the first mentioned powder metallurgy technique, and in addition requires close control over a complex series of chemical reactions and manipulative steps.

The simplest, most convenient and most reproducible technique for the dispersion hardening of a metal involves first forming an alloy of the parent metal with a small but significant quantity of an alloying metal which will form a hardened metal oxide, followed by subjecting the metal alloy to an environment containing elemental oxygen in order to diffuse oxygen into the solid metal alloy matrix and thereby react with the allowing metal to form minute particles of a stable hardening metal oxide dispersed throughout the metal matrix. The resulting dispersion hardened metal has greatly increased strength, rigidity and hardness as compared to the non-hardened metal, and the hardened metal retains its strength and hardness at temperatures which would cause precipitation hardened metals to soften and lose their strength.

The actual treatment of the metal to obtain the desired hardening metal oxide dispersed therein has heretofore been a complicated and difiicult procedure. For example, in the conventional procedure for dispersion hardening of copper, substantially pure copper is alloyed with a small but significant quantity of a metal which will form a hardening metal oxide. The alloy is then cast or otherwise formed into a suitable ingot or a finished shape, and the solid copper alloy is placed in a furnace containing elemental oxygen at about one atmosphere pressure for a sufficient length of time to form a coating of cuprous oxide on the outer surface of the alloy. The oxygen atmosphere is then replaced with nitrogen or another inert atmosphere, and the cuprous oxide coating is decomposed so as to release oxygen atoms which diffuse into the interior of the alloy where they react with the alloying metal to form hardening metal oxides dispersed throughout the copper matrix and which thereby harden and strengthen the copper. Accurate control of the partial pressure of oxygen in the ambient atmosphere is of vital importance in this process. That is to say, the partial pressure of oxygen should be as high as possible to assure good diffusion of oxygen into the metal matrix, but at the same time it should not be so high as to oxidize the copper or parent metal in addition to the alloying metal. Specifically, the partial pressure of oxygen should almost equal but should not exceed the decomposition oxygen pressure of the oxide of the parent metal. However, the decomposition pressure of the parent metal oxide is a function of temperature. Therefore, the oxygen partial pressure must be closely controlled to correspond to variations in temperature in order to assure maximum effective diffusion of oxygen into the parent metal.

We have now discovered that the partial pressure of oxygen can be automatically controlled and the maximum effective diffusion of oxygen into the parent metal can be obtained without deleterious oxidation of the parent metal by a novel process which employs a powdered oxide of the parent metal as the source of the oxygen diffused into the metal matrix. More specifically, we have found that if the metal alloy to be hardened (for example, copper containing a small but significant amount of aluminum as the hardening metal of the alloy) is packed in a finely divided powder of the lowest valent oxide of the parent metal (i.e. cuprous oxide, in the case of copper) and then is heated at a temperature of at least about 750 C. for a period of time sufficient to insure the desired degree of diffusion of oxygen into the parent metal, the correct partial pressure of oxygen will automatically be maintained so as to obtain reaction of oxygen with the hardening metal of the alloy but not with the parent metal itself. As a result, minute particles of a hardening metal oxide are formed dispersed through the matrix of the parent metal, and a high quality dispersion hardened metal product is obtained.

Dispersion-hardened metal produced by our process is many times stronger than the non-hardened parent metal, and the dispersion hardened metal product retains its strength and hardness even when heated to tempera tures which would anneal work-hardened or reprecipitate the intermetallic hardening compounds of age-hardened metals. Moreover, we have found that the strength of dispersion hardened metal produced in accordance with our process may be further significantly increased by subjecting the dispersion hardened metal to cold deformation (for example, by means of cold-rolling), and, contrary to what one skilled in the art would expect, we have found that the cold-worked dispersion hardened metal thus obtained can be heated to the annealing temperature of the parent metal without reducing the strength of the cold-worked and annealed product to that of the original dispersion-hardened metal product. That is to say, the strength of a dispersion-hardened metal produced in accordance with our invention can be significantly increased by cold working the metal, and a significant portion of this increase in strength is retained even when the cold worked product is heated to the annealing temperature of the parent metal.

Although our new process is applicable to a variety of non-ferrous metals which may be hardened by dispersion-hardening procedures, it will be described herein specifically in conjunction with the dispersion hardening of copper. The usual hardening metal oxides employed to harden copper in the dispersion hardening process are aluminum oxide, beryllium oxide, chromium oxide, magnesium oxide and zirconium oxide, these refractory metal oxides being formed in situ in the copper matrix in the manner described herein. As previously mentioned, an alloy of copper with a small but significant quantity of a metal capable to forming a hardening metal oxide is first prepared in the usual manner. The amount of the hardening metal constituent present in the copper alloy is determined by the quantity of the hardening metal oxide it is desired to be formed dispersed throughout the copper matrix, and generally is less than about 1% by weight of the alloy. For example, a copper base alloy containing about 0.50% by weight of aluminum is an excellent starting material for the production of dispersion hardened copper in accordance with our process.

The solid copper alloy is subjected to internal oxidation in accordance with our process by packing the alloy in finely divided cuprous oxide powder contained. in a container formed of copper or other suitable material. The copper alloy packed in the cuprous oxide powder is then heated at a temperature of at least about 750 C. and below the softening temperature of the copper alloy, and preferably between about 800 and 950 C., for a period of time suflicient to insure the desired degree of diffusion of elemental oxygen into the copper matrix and thereby form minute particles of the desired hardening metal oxide dispersed throughout the copper matrix. The length of time that the copper alloy is maintained at the diffusion or reaction temperature is dependent upon the size (for example, the thickness) of the copper part being treated, the amount of the hardening metal constituent present in the alloy and the degree of penetration by dilfusion of oxygen desired. For example, a copper alloy containing 0.44% aluminum which is'packed in cuprous oxide in accordance with our invention and heated at a temperature of 750 C. for a period of hours will have a dispersion hardened layer about 0.002 inch in depth formed adjacent the outer surfaces thereof, a similar copper alloy packed and heated at a temperature of 825 C. for a period of 5 hours will have a layer of dispersion hardened copper about 0.010 inch in depth formed adjacent the surfaces thereof, and the same copper alloy packed and heated at a temperature of 950 C. for a period of 2 hours will have a layer of dispersion hardened copper about 0.020 inch in depth formed adjacent the surfaces thereof. In a convenient adaptation of our process, the copper alloy part to be hardened is packed in cuprous oxide powder contained in a suitable container and is placed in a suitable furnace for over-night heating at the desired temperature.

Our new process avoids all of the complications of control of the partial pressure of oxygen inherent in prior art processes, the partial pressure being automatically increased or decreased with fluctuations in temperature so as to insure maximum diffusion of oxygen into the parent metal matrix while avoiding internal oxidation of the parent metal. As noted, this control of the partial pressure of oxygen is obtained by using the lower valent oxide of the parent metal (which, in the case of copper, is cuprous oxide), it having been found by experience that the higher valent oxide (that is, cupric oxide) gives a higher than necessary oxygen pressure and thereby oxidizes some of the copper into which it diffuses. As each oxygen atom from the cuprous oxide powder surounding the copper part enters the copper lattice it is scavenged by an atom of the alloying metal with the resulting formation of fine particles of the hardening metal oxide dispersed throughout the copper matrix. Despite any subsequent heat treatment, the hardening metal oxide does not dissociate but remains an effective dispersion hardening agent. As a result, the elastic limit or strength of the hardened copper is increased to about 12,000 to 15,000 p.s.i. or in the order of about seven times over that of non-hardened high purity copper, and this increase in strength is retained even when the dispersion hardened copper is heated to temperatures which would soften other coppers such as work hardened or precipitation hardened coppers.

The strength of the dispersion-hardened copper may be even further increased by cold-working the dispersionhardened product, and the increase in the strength thus obtained is substantially retained even when the cold-worked dispersion-hardened copper is heated to above the annealing temperature of copper. Thus, dispersion-hardened copper produced in accordance with our process having an elastic limit of about 12,000 to 15,000 p.s.i. may be cold-worked, for example, by cold rolling to reduce the cross sectional area by about 50%, to obtain a coldworked product having an elastic limit of about 40,000 p.s.i. Heating the cold worked product to above the annealing temperature of copper reduces the elastic limit of the product to around 22,000 to 24,000 p.s.i., or nearly twice the elastic strength of the original dispersion-hardened copper. Moreover, the elastic limit of the annealed, cold-worked, dispersion hardened copper is more than ten times the elastic limit of high purity copper and more than four times the elastic limit of precipitation hardened copper subjected to the same cold working and heat treatment.

The following examples are illustrative but not limitative of the practice of our invention.

Example I A plate inch thick of a zirconium copper alloy containing 0.15% zirconium and the balance essentially copper was packed in fine cuprous oxide powder in a copper container, and the thus packed copper alloy plate was heated at a temperature of 950 C. for a period of one hour. The copper alloy had a Rockwell F hardness of 40 and a yield strength of 6000 p.s.i. prior to the oxygen treatment, and the dispersion hardened plate had a Rockwell F hardness of and a yield strength of 20,000 p.s.i. following the oxygen treatment.

Example II A test strip of an aluminum copper alloy containing aluminum and the balance essentially copper was packed in cuprous oxide powder and then heated at a temperature of 750 C. for a period of 16 hours. The copper alloy had a Rockwell F hardness of 40 and a yield strength of 6000 p.s.i. prior to the oxygen treatment,

and the dispersion-hardened strip had a Rockwell F hardness of 80 and a yield strength of 20,000 p.s.i. following the oxygen treatment. The depth of the dispersion hardened layer adjacent the surfaces of the strip was 0.003 inch.

Example III A test strip of an aluminum copper alloy containing 0.44% aluminum and the balance essentially copper was packed in cuprous oxide powder and then heated at a temperature of 825 C. for a period of 7 hours. The copper alloy had a Rockwell F hardness of 40 and a yield strength of 6000 p.s.i. prior to the oxygen treatment and the dispersion-hardened strip had a Rockwell F hardness of 80 and a yield strength of 20,000 p.s.i. following the oxygen treatment. The depth of the dispersion hardened layer adjacent the surfaces of the strip was 0.011 inch.

Example IV Example V A test strip of an aluminum copper alloy containing 0.25% aluminum and the balance essentially copper was packed in cuprous oxide powder and then heated at a temperature of 825 C. for a period of 7 hours. The copper alloy had a Rockwell F hardness of 40 and a yield strength of 6000 p.s.i. prior to the oxygen treatment and the dispersion-hardened strip had a Rockwell F hardness of 80, an elastic limit of 12,000 p.s.i. and a yield strength of 20,000 p.s.i. following the oxygen treatment.

Example VI A test strip of an aluminum copper alloy containing 0.57% aluminum and the balance essentially copper was packed in cuprous oxide powder and then heated at a temperature of 825 C. for a period of 7 hours. The copper alloy had a Rockwell F hardness of 40 and a yield strength of 6000 p.s.i. prior to the oxygen treatment and the dispersion-hardened strip had a Rockwell F hardness of 80,

an elastic limit of 12,000 p.s.i. and a yield strength of 20,000 p.s.i. following the oxygen treatment.

Example VII A test strip of dispersion hardened copper produced in accordance with our process having an elastic limit of 12,000 p.s.i. was subjected to cold rolling to reduce the cross sectional area by 50%. The resulting cold rolled copper test strip had an elastic limit of 39,000 p.s.i. The test strip was then heated to a temperature of 1000 C., and following this heat treatment it was found to have an elastic limit of above 22,000 p.s.i. By Way of contrast, the elastic limit of pure copper may be raised from about 2400 p.s.i. to about 38,000 p.s.i. by cold working, but annealing of the cold worked copper will reduce the elastic limit once again to about 2400 p.s.i. Similarly, the elastic limit of precipitation hardened copper may be increased to the order of about 40,000 p.s.i. by cold working, but heating of the cold Worked precipitation hardened copper to about 1000 C. will reduce its elastic limit to the order of about 5000 p.s.i.

From the foregoing description of our process for the dispersion hardening of metal, it will be seen that we have made an important contribution to the art to which our invention relates.

What is claimed is:

1. A copper alloy in the cold-worked annealed condition having a hardening oxide metal dispersion therein produced by forming an alloy of the copper to be hardened with an alloying metal selected from the group consisting of aluminum, chromium, magnesium and zirconium in an amount sufficient to form dispersed oxides, oxidizing said copper alloy in a pack of C11 0 at a temperature of at least 750 C. to form minute particles of the hardening metal oxide dispersed throughout the matrix of the copper alloy, cold working said alloy to reduce the cross sectional area by about 50% followed by annealing at a temperature of about 1000" C., said alloy being characterized by having an elastic strength of about 12,000 p.s.i. prior to cold working, and an elastic strength after cold working and annealing of at least about 22,000 p.s.i.

References Cited UNITED STATES PATENTS 2,493,951 l/ 1950 Druyvesteyn et al. 14813.2 3,117,894 1/1964 COXe l481l.5

FOREIGN PATENTS 654,962 7/ 1951 Great Britain.

OTHER REFERENCES Rhines et al., Rates of High Temperature Oxidation of Dilute Copper Alloys, AIME Transactions, vol. 147, pp. 205-219.

CHARLES N. LOVELL, Primary Examiner. 

